A conventional power steering system normally includes a power steering pump, power steering gear, power steering reservoir, supply hose, pressure hose, and return line. Turning of the steering wheel is converted into linear travel by the steering gear through the meshing of the helical teeth of a pinion with the teeth of a rack. Power assist is provided by a rotary type control valve which directs oil from the power steering pump to either side of a piston carried by the rack.
Road feel is controlled by the diameter of a torsion bar which initially steers the vehicle. As steering effort increases, as in a turn, the torsion bar twists, causing relative rotary motion between the rotary valve body and valve spool of the control valve. This movement directs oil behind the rack piston, which in turn builds up hydraulic pressure and assists in the turning effort.
Hydraulic pressure is delivered to the control valve and the power steering gear preferably by a vane type pump. The power steering pump is connected to the control valve by a pressure hose and a return hose.
During certain driving maneuvers, and dependent on particular vehicle suspension designs, various road inputs, such as pot holes and the like, can be transmitted from the road wheels through the steering gear and steering column to the steering wheel. The driver feels vibrations and may hear a knock-like noise. This phenomenon, commonly known as "rack rattle", may be more pronounced at different speeds or different maneuvers.
There is a need for something to dampen these road inputs. In accordance with this invention, a flow restriction device is provided in the return line. Restricting the return flow of fluid has been found to be highly effective in suppressing the transmission of road inputs through the steering gear back to the steering wheel.
In accordance with the preferred embodiment disclosed herein, the restriction device comprises a tubular formation having a cylindrical wall provided with a nose portion at the downstream end which tapers radially inwardly in the direction of return flow of fluid and terminates in an outlet orifice of reduced internal cross-section. Preferably, the return line comprises a relatively rigid tube extending from the control valve having the tubular formation integrally formed on its end, and a flexible tube extending from the reservoir and forced over the tubular formation in the manner of a sleeve. Preferably, the tubular formation has spaced apart enlargements and a band encircles the flexible tube between the enlargements to retain the flexible tube sleeved on the rigid tube.
An object of this invention is to provide a means for suppressing rack-rattle in a power steering system having the foregoing characteristics and capabilities.
Another object is to provide a restriction device for suppressing rack rattle which is of simple and rugged construction, and capable of being readily and easily installed.
These and other objects, features and advantages of the invention will become more apparent as the following description proceeds, especially when considered with the accompanying claims and drawings.